Pipets containing electrolyte and electrodes

Abstract

Apparatus comprises a pipet comprising first and second channels separated by a septum and having a tip at which each of the first and second channels is open; an electrolyte solution contained within both the first and second channels of the pipet; a first electrode extending into the electrolyte in the first channel of the pipet; a second electrode extending into the electrolyte in the second channel of the pipet; means for applying a potential difference between the first and second electrodes; means for measuring alternating current components of a current flowing to or from the first electrode; means for applying an oscillatory perturbation to the pipet; and means responsive to the alternating current measured to be flowing to or from the first electrode to adjust a position of the pipet such as to control a separation between the tip of the pipet and a surface of interest.

The invention claimed is: 
     
       1. A method comprising:
 providing a pipet comprising first and second channels separated by a septum and having a tip at which each of the first and second channels is open; 
 providing an electrolyte solution within both the first and second channels of the pipet; 
 providing a first electrode extending into the electrolyte in the first channel of the pipet; 
 providing a second electrode extending into the electrolyte in the second channel of the pipet, the second electrode at a ground potential; 
 applying a potential difference between the first and second electrodes; 
 applying an oscillatory perturbation to the pipet at a perturbation frequency; 
 measuring alternating current components of a current flowing to or from the first electrode; and 
 responding to alternating current at the perturbation frequency or at a harmonic of the perturbation frequency measured to be flowing to or from the first electrode by adjusting a position of the pipet so as to maintain a separation between the tip of the pipet and a surface of interest. 
 
     
     
       2. A method as claimed in  claim 1 , wherein the electrolyte solution is an aqueous solution. 
     
     
       3. A method as claimed in  claim 1  further comprising measuring a direct current component of the current flowing to or from the first electrode. 
     
     
       4. A method as claimed in  claim 1  further comprising measuring a current flowing to or from a substrate supporting the surface of interest. 
     
     
       5. A method as claimed in  claim 1  wherein responding to the alternating current measured to be flowing to or from the first electrode by adjusting the position of the pipet so as to maintain the separation between the tip of the pipet and the surface of interest comprises responding to a component of the alternating current at the perturbation frequency detected to be flowing to or from the first electrode by adjusting the position of the pipet such as to maintain the separation between the tip of the pipet and the surface of interest. 
     
     
       6. Apparatus comprising:
 a pipet comprising first and second channels separated by a septum and having a tip at which each of the first and second channels is open; 
 an electrolyte solution contained within both the first and second channels of the pipet; 
 a first electrode extending into the electrolyte in the first channel of the pipet; 
 a second electrode extending into the electrolyte in the second channel of the pipet, the second electrode connected to ground; 
 a voltage source configured to apply a potential difference between the first and second electrodes; 
 a mechanical oscillator configured to apply an oscillatory perturbation to the pipet at a perturbation frequency; 
 a current meter configured to measure alternating current components of a current flowing to or from the first electrode; and 
 a pipet adjustment device responsive to alternating current at the perturbation frequency or at a harmonic of the perturbation frequency detected, by the current meter, to be flowing to or from the first electrode to adjust a position of the pipet so as to control a separation between the tip of the pipet and a surface of interest. 
 
     
     
       7. Apparatus as claimed in  claim 6 , wherein the electrolyte solution is an aqueous solution. 
     
     
       8. Apparatus as claimed in  claim 6 , wherein the current meter is configured to measure a direct current component of the current flowing to or from the first electrode. 
     
     
       9. Apparatus as claimed in  claim 6 , wherein the current meter is configured to measure a current flowing to or from a substrate on which the surface of interest is supported. 
     
     
       10. Apparatus as claimed in  claim 6 , wherein the pipet adjustment device is responsive to the alternating current detected to be flowing to or from the first electrode to adjust a position of the pipet so as to maintain a separation between the tip of the pipet and a surface of interest is responsive to a component of the alternating current at the perturbation frequency. 
     
     
       11. Apparatus as claimed in  claim 10 , wherein the pipet adjustment device is responsive to the alternating current detected to be flowing to or from the first electrode to adjust a position of the pipet so as to maintain a separation between the tip of the pipet and a surface of interest is configured to adjust the height of the pipet so as to maintain a constant or substantially constant amplitude of the component of the alternating current. 
     
     
       12. Apparatus as claimed in  claim 6 , wherein the pipet adjustment device is responsive to the alternating current detected to be flowing to or from the first electrode to adjust a position of the pipet so as to adjust or maintain a separation between the tip of the pipet and a surface of interest is responsive to a component of the alternating current at a harmonic of the perturbation frequency. 
     
     
       13. Apparatus as claimed in  claim 6 , comprising a translator for translating the tip of the pipet relative to the surface of interest. 
     
     
       14. Apparatus as claimed in  claim 6 , comprising a translator for translating the tip of the pipet relative to the surface of interest in a unilateral linescan motion. 
     
     
       15. Apparatus as claimed in  claim 6 , comprising a user-operable position adjuster for adjusting the position of the pipet prior to operation of the pipet adjustment device to adjust the position of the pipet so as to maintain the separation between the tip of the pipet and the surface of interest. 
     
     
       16. Apparatus as claimed in  claim 6 , wherein the mechanical oscillator is configured to apply a sinusoidal perturbation to the pipet. 
     
     
       17. Apparatus as claimed in  claim 6 , wherein the mechanical oscillator is configured to apply a perturbation to the pipet at a frequency between 5 and 100 000 Hz. 
     
     
       18. Apparatus as claimed in  claim 6 , wherein the mechanical oscillator is configured to apply a perturbation to the pipet at an amplitude between 1 nm and 1 μm. 
     
     
       19. Apparatus as claimed in  claim 6 , wherein the current meter is configured continuously to monitor current flowing to or from the first electrode.